Brexit to require parliamentary approval in setback for Theresa May

Parliament alone has the power to trigger Brexit by notifying Brussels of the UK’s intention to leave the European Union, the high court has ruled.

The judgment, delivered by the lord chief justice, Lord Thomas of Cwmgiedd, is likely to slow the pace of Britain’s departure from the EU and is a huge setback for Theresa May, who had insisted the government alone would decide when to trigger the process.

The lord chief justice said that “the most fundamental rule of the UK constitution is that parliament is sovereign”.

A government spokesman said that ministers would appeal to the supreme court against the decision. The hearing will take place on 7-8 December.

The lord chief justice said: “The court does not accept the argument put forward by the government. There is nothing in the 1972 European Communities Act to support it. In the judgment of the court the argument is contrary both to the language used by parliament in the 1972 act, and to the fundamental principles of the sovereignty of parliament and the absence of any entitlement on the part of the crown to change domestic law by the exercise of its prerogative powers.”

Unless overturned on appeal at the supreme court, the ruling threatens to plunge the government’s plans for Brexit into disarray as the process will have to be subject to full parliamentary control.

Government lawyers had argued that prerogative powers were a legitimate way to give effect “to the will of the people” who voted by a clear majority to leave the European Union in the June referendum.

But the lord chief justice declared: “The government does not have power under the crown’s prerogative to give notice pursuant to article 50 for the UK to withdraw from the European Union.”

The international trade secretary, Liam Fox, said the government was disappointed by the high court decision but added that “the government is determined to respect the result of the referendum”.

Ukip’s leader, Nigel Farage, said he was angered at the decision. “I worry that a betrayal may be near at hand … I now fear that every attempt will be made to block or delay the triggering of article 50. If this is so, they have no idea of the level of public anger they will provoke.”

But the Labour leader, Jeremy Corbyn, said: “This ruling underlines the need for the government to bring its negotiating terms to parliament without delay. Labour respects the decision of the British people to leave the European Union. But there must be transparency and accountability to parliament on the terms of Brexit.”

The Lib Dem leader, Tim Farron, said he was delighted by the ruling. “Given the strict two-year timetable of exiting the EU once article 50 is triggered, it is critical that the government now lay out their negotiating to parliament, before such a vote is held,” he said.

By handing responsibility for initiating Brexit over to MPs, the three senior judges – Lord Thomas; the master of the rolls, Sir Terence Etherton, and Lord Justice Sales – have ventured on to constitutionally untested ground.

The legal dispute focused on article 50 of the treaty on European Union, which states that any member state may leave “in accordance with its own constitutional requirements” – an undefined term that has allowed both sides to pursue rival interpretations.

The decision may undermine the prime minister’s authority in conducting negotiations with other EU states in the run-up to the UK’s withdrawal.

Gina Miller, the lead claimant in the case, said: “It was the right decision because we were dealing with the sovereignty of parliament. It was not about winning or losing. It was about what was right. Now we can move forward with legal certainty.”

Deir Dos Santos, a hairdresser and the other lead claimant, said: “Today’s judgment is a victory for everyone who believes in the supremacy of our parliament and the rule of law. I have never challenged the result of the referendum – in fact I voted for Brexit for the sole reason that I wanted power to be returned from Europe to the British parliament. But I did not think it was right for the government then just to bypass parliament and try to take away my legal rights without consulting parliament first.”

John Halford, the solicitor at Bindmans who represented the People’s Challenge group, said: “The oversight, control and democratic accountability needed for decisions on Brexit have to match the consequences of those decisions for UK citizens. That is why our constitution empowers parliament, not the government, to take decisions.”

John Shaw, chair of the organisation Fair Deal for Expats, said: ”This is superb news. We were convinced that our case was just. We’re delighted that the court agrees with us. There now needs to be a proper debate about how the rights of expats will be protected.”

The court took barely two and a half weeks to deliver its judgment. Lawyers for the government had argued that the prime minister has authority under the royal prerogative – its executive powers – to give formal notification.

The challengers maintained that parliamentary approval and legislation was required for such a fundamental change that would deprive millions of UK citizens of their legal rights. If ministers alone triggered Brexit they would be undermining the sovereignty of parliament, it was argued.